High-energy astrophysics: Neutron stars and black holes

When a star that is about eight times more massive than the Sun consumes all the fuel in its nucleus, gas pressure in the core suddenly drops, there is no longer a force that balances gravity, and the outer layers of the star collapse under their own weight. The collapsed material bounces back, and eventually is expelled in a spectacular explosion called type II supernova. At the centre of the explosion, there remains a compact object, either a neutron star or a black hole.

Neutron stars: Neutron stars have radii of about 10 to 15 km, and masses of about 1 to 2 solar masses. At their interior density can reach up to 10-15 times nuclear density at saturation, ρ₀ = 2.8 * 10¹⁴ gr cm^-3. At densities of about 1/2 ρ₀, protons and neutrons are no longer bound to nuclei and they form a non-ideal liquid. The equation of state, the relation between pressure and energy density at the interior of the neutron star, and the composition of nuclear matter at densities above ~ 2 ρ₀ are rather uncertain. For densities below ρ₀ matter consists mostly of nucleons (protons and neutrons) plus electrons and muons. Since the chemical potential of electrons and neutrons increase with density, heavier particles, e.g. hyperons (hadrons having a strange quark), can in principle be produced when density increases toward the neutron star centre. At high densities, π and K mesons (pions and kaons; a quark-antiquark pair) may also appear; the core of the neutron star could then consist of pion or kaon Bose-Einstein condensates. At extreme densities it may be energetically more favourable to have a fluid made of deconfined up (u), down (d) and strange (s) quarks.

Laboratory measurements on Earth allow us to constrain the equation of state up to ρ ~ ρ₀. Since the structure of neutron stars is set by the equation of state of matter at these high densities, measurements of neutron-star masses and radii can provide information not only on the physical makeup of neutron stars, but also on the nature of the interactions between the particles at those densities. The figure below shows the neutron star mass-radius relation calculated for some representative equations of state (see caption for explanations). Each curve is parametrised in terms of the central density of the neutron star. If nuclear physics is correct, a measurement of M and R for a neutron star allows us to determine its internal composition.

Very accurate mass measurements of neutron stars in binary pulsar systems yield < M > ~ 1.35 M_sun. Unfortunately, this range of masses, indicated by the two horizontal lines in the figure above, does not set any useful constrain on the equation of state. To be able to distinguish between different equations of state we need a measurement of the neutron star radius, or a simultaneous measurement of mass and a combination of mass and radius. X-ray observations offer essentially the only way to measure R for a neutron star.

Black holes: Neutron stars are supported against gravity by pressure from a degenerate neutron gas. There is an upper limit to the mass of a neutron-degenerate object. The actual value of this upper limit depends on the equation of state of nuclear matter. When the mass of the star is larger than that limit, neutron-degenerate pressure can no longer balance gravity, and the star may collapse into a black hole.

Black hole are the most compact objects that exist, and the ones with the strongest gravitational fields. Gravity is so strong that even light cannot escape from them. Therefore, classical black holes are undetectable through electromagnetic radiation. (Quantum-mechanical effects, however, allow black holes to emit, via the so called Hawking radiation.)

Due to their strong gravitational attraction, black holes can nevertheless be detected if they interact with their environment. For instance, if a member of a close binary system is a black hole, gas could escape from the normal star, form a disc around the black hole, and eventually accrete onto the black hole. Matter in the accretion disc becomes so hot that it radiates in the X-ray band. Observations of this accretion disc in the X-ray band provide information of the dynamics in the vicinity of the black hole.

Neutron stars and black holes are the densest objects in the Universe. The spectrum and time variability of radiation from matter near neutron stars and black holes show the imprint of the curvature of space-time as predicted by general relativity. This has strong implications for astrophysics and cosmology in general

Some of the questions that one wants to answer are:

• What are the properties of matter under the extreme conditions prevalent in the interior of a neutron star?
• What are the observational signatures of black holes?
• Can we observationally verify the extraordinary predictions of General Relativity for the properties of curved space-time near these objects?
• How do particles and radiation behave near these compact objects?

Our focus is on the following topics:

• Physics of accretion onto neutron stars and black holes.
• Effects of extremely strong gravitational fields.
• Internal composition of neutron stars.

Members of the group:

Mendez, Hiemstra, Zhang